The high voltage, fast epitaxial rectifier is a P-N junction diode of monocrystalline silicon in which there have been included one or more thin layers that are germanium-doped. The germanium introduces strain in the crystal that serves to speed up the recombination of charge carriers in the depletion layer of the diode so as to increase the speed with which the diode can be switched from a low-impedance state to a high-impedance state as the voltage across the junction switches from a forward bias to a reverse bias. Diodes of this kind are described in U.S. Pat. No. 5,097,308 (issued on Mar. 17, 1992), U.S. Pat. No. 5,102,810 (issued on Apr. 7, 1992), U.S. Pat. No. 5,298,457 (issued on Mar. 29, 1994), and U.S. Pat. No. 5,342,805 (issued on Aug. 30, 1994).
In high voltage rectifiers it is very important to avoid excessive heating. This is best avoided by providing that the diode exhibits a low voltage drop state when biased in the forward direction and a high voltage state when biased in the reverse direction and that the transition between the two states occurs rapidly. As is known, one of the problems characteristic of such diodes is that when the bias shifts from the forward direction to the reverse direction, charge carriers that have diffused into the high resistivity depletion region of the junction during the forward bias state remain in the high ohmic region. These charge carriers tend to persist for a finite time, usually termed either the recombination time or the minority charge carrier lifetime, and while they persist, the diode remains in a conductive state even though the bias is in the reverse direction. This gives rise to leakage currents that flow in the direction opposite to that desired and these currents can be a major problem. To overcome this problem, it is known to include in the depletion region impurities or crystal imperfections that serve as lifetime killers. As mentioned above, the inclusion of doped germanium layers, typically in the middle of the depletion region, has been used previously to introduce these crystal imperfections that serve this role. Additionally, as described in the aforementioned patents, it is known also to include gold and platinum atoms in the depletion region to augment the role of the doped-germanium layers in this role.
It is also known to be important in high voltage rectifiers that employ silicon diodes to insure that any breakdown of the rectifying junction that may occur, usually because the reverse bias is temporarily excessive because of transients or surges in the power supply, occur in the interior or bulk of the diode rather than at its surface. This is because the interior is more rugged and, accordingly, breakdowns localized in the interior are much less likely to do permanent damage to the diode than are breakdowns at a surface of the diode. To this end, various expedients have been employed, including the use of passivating layers to cover the regions where rectifying junctions intersect the surface or the use of special geometries, such as mesas with tapered side walls, as described in U.S. Pat. No. 5,102,810 (issued on Apr. 7, 1992).